Improvements in therapy are aimed at better diagnosis and more effective treatment. The use of computer simulation has the potential to improve therapy in both ways. Computational methods have been used extensively in diagnosis, for interpreting MRI results, CAT scans and the development of treatments in the study of biochemical structure. The present review examines how simulation may be used to play a more fundamental role in therapeutic treatment often referred to as In Silico Biology. Simulation of cells, organs and systems, such as pulmonary and cardiovascular, can be used in clinical practice to improve diagnosis. Initially these developments will probably have to be refined before being used in general practice. However, when these simulations are linked with artificial intelligence techniques containing experimental databases then very powerful tools will result. The use of simulation in the design and development of new drugs can both be cost and time effective. Again, artificial intelligence techniques are likely to play an important role in evaluating patient risk factors and unwanted side effects.